1. Field of the Invention
The present invention relates to a reflection liquid crystal display device having a liquid crystal enclosed between a pair of substrates opposed to each other and to a liquid crystal display system using the liquid crystal display device.
2. Description of the Prior Art
Liquid crystal display devices are used in a wide range of apparatuses from an electronic calculator to in a portable TV. A display device, which especially requires clear colors, generally employs an active matrix liquid crystal display device equipped with a switching transistor and the like for each pixel.
Representative display modes used in the active matrix liquid crystal display device include the twisted nematic mode, the dynamic scattering mode, and the guest-host mode. Hereinafter, the liquid crystal display device will be referred to as the "LCD device", the twisted nematic mode as the "TN mode", the dynamic scattering mode as the "DS mode", and the guest-host mode as "GH mode".
In the TN mode, the liquid crystal cell interposed between a pair of polarizing plates includes liquid crystal molecules which are twisted at 90.degree. as the initial alignment. An image is displayed utilizing optical characteristics of the cell, namely, the optical rotatory polarization under no electric field and the depolarization under an electric field.
The TN mode has the following problems. Since the polarization of light is utilized, only 50% or lower ratio of light emitted from a light source is effectively used, thereby darkening the displayed image. Further, an alignment film is required. Static electricity generated by rubbing the alignment film destroys the transistor and attracts dust thereto.
In the DS mode, a voltage higher than a certain level is applied to the liquid crystal cell to scatter the liquid crystal molecules, thereby scattering the light incident on the liquid crystal display device. An image is displayed utilizing the scattered light.
The DS mode has problems of a low resistance of the liquid crystal, a large power consumption, and a low speed of response (50 to 100 ms) at the time of voltage supply.
In the GH mode, a liquid crystal material including a dichroic coloring matter is used. Utilizing the phenomenon that the direction of alignment is changed by the application of an electric field, the direction of alignment of the colored liquid crystal molecules is changed, thereby changing the color of the liquid crystal cell. An image is displayed by such change of the color of the liquid crystal cell.
The GH mode has problems of dark display to causing a low contrast (approximately 3 to 5) due to a display principle thereof and a low speed of response (100 to 200 ms).
In general, a liquid crystal display device includes a glass substrate and a thin film transistor for controlling display. Characteristics of the transistor depend on the material of the thin film. Usually, the thin film is formed of amorphous silicon, low-temperature polysilicon or high-temperature polysilicon. Hereinafter, the thin film transistor will be referred to as the "TFT".
An amorphous silicon TFT is formed by CVD or sputtering. Since the amorphous silicon film can be formed at a low temperature of approximately 350.degree. C. or lower, an inexpensive ordinary glass, for example, Corning 7059 produced by Corning Inc. can be used as the substrate. Moreover, a relatively large display device which is up to 15 inches across can be realized. For these advantages, most active matrix LCD devices employ the amorphous silicon TFT.
The amorphous silicon TFT has the following problems:
(1) Due to a great number of traps of the amorphous silicon, the thin film has an electric field mobility of 1 cm.sup.2 V.sup.-1 S.sup.-1 or less. A transistor having such a thin film has a large resistance at ON.
(2) Since the ordinary glass used for the substrate cannot be processed at a high temperature of 600.degree. C. or higher, it is impossible to use a thermal oxide film, which has a high breakdown voltage and hardly generates any pin holes. Therefore, in the case when a driving circuit or the like requires a complicated structure and excellent performance, such a circuit cannot be mounted on the same substrate as a TFT.
A polysilicon TFT has a higher electric field mobility than that of the amorphous silicon TFT. Owing to a polysilicon thermal oxide film used as a gate oxide film constituting the TFT, the TFT has high transistor characteristics. Further, since the polysilicon TFT can be self-aligned using an ion implantation technology, a simple driving circuit can be formed integratedly with the TFT. However, since the production of a high quality polysilicon requires a high processing temperature of 600.degree. C. or higher, expensive quartz glass is used for the substrate. Therefore, the polysilicon TFT is mainly used for an LCD device having a relatively small display area such as the one in a viewfinder of a video tape recorder. Examples of the polysilicon TFT include a low-temperature polysilicon TFT and a high-temperature polysilicon TFT.
The whole process of producing the low-temperature polysilicon TFT is conducted at 550.degree. to 600.degree. C., using a heat resistant glass as the substrate. If necessary, annealing is conducted for a long period of time or recrystallizing by the use of laser is conducted. This type of TFT, which has an electric field mobility of 50 cm.sup.2 V.sup.-1 S.sup.-1 in .mu..sub.e (electronic mobility) and 15 cm.sup.2 V.sup.-1 S.sup.-1 in .mu..sub.h (hole mobility), generally has better characteristics than those of the amorphous silicon TFT.
The high-temperature polysilicon TFT is mounted on a highly heat-resistant quartz substrate. Since the production of the high-temperature polysilicon TFT is conducted at a high temperature of up to approximately 1,200.degree. C., almost all the processes used for producing an integrated circuit (hereinafter, referred to as the "IC") can be used. This type of TFT, which has an electric field mobility of 100 to 400 cm.sup.2 V.sup.-1 S.sup.-1 in .mu..sub.e and 50 to 150 cm.sup.2 V.sup.-1 S.sup.-1 in .mu..sub.h, has the best characteristics of the three types of TFTs mentioned above.
The polysilicon TFT has better characteristics than those of the amorphous silicon TFT as mentioned above. Therefore, a driving circuit can be mounted on the same substrate as the TFT.
However, the polysilicon TFT has a low speed of response. For an experiment, complementary metal oxide semiconductor (hereinafter, referred to as "CMOS") shift registers were produced of a low-temperature polysilicon TFT and a high-temperature polysilicon TFT. Under a voltage of 15 V, the shift register produced of the low-temperature polysilicon TFT showed a maximum operating frequency of approximately 5 MHz, and the shift register produced of a high-temperature polysilicon TFT showed that of approximately 15 MHz. These values are sufficient for a shift register for processing, for example, 2,000 scanning lines at 60 Hz, but are not sufficient for a data driver of an LCD device for driving 2,000.times.2,000 pixels at 60 Hz.
The polysilicon TFT has a large leak current. Therefore, an increase of the ON/OFF ratio requires increasing the size of the transistor or connecting the transistors in series. Accordingly, in the case when a driving circuit, a switching circuit or the like requires a complicated structure, such a circuit cannot be mounted on the same substrate as the TFT. Moreover, the polysilicon TFT having the above inconvenience is not suitable to a certain type of liquid crystal material which requires a high voltage for driving. A high quality display cannot be realized in the case when a liquid crystal material which has a low resistance or requires a high voltage for switching is used.
The driving circuit, a memory circuit, a logic circuit and other circuits necessary for the LCD device are formed by a method for mounting IC chips having such functions on the substrate, namely, by a tape automated bonding method (hereinafter, referred to as the "TAB method" or a chip on glass method (hereinafter, referred to as the "COG method"). These methods will be described in detail with examples.
The TAB method is the main method for connecting a driving IC chip to a high precision LCD panel. FIGS. 1a through 1c illustrate an example of the TAB method. As is shown in FIG. 1a, connecting terminals 21 are formed on an electrode of a driving IC chip 20. Each connecting terminal 21 is formed of gold and has a height of approximately 20 .mu.m. A tape carrier 22 has a film 22a formed of polyimide or the like, and a connecting terminal 22b formed of a copper wiring coated with gold or tin on the film 22a. The connecting terminals 21 and 22b are positionally aligned and connected by thermo compression bonding using a heating jig 23. Then, the portion of the IC chip 20 exposed to the thermo compressing bonding is sealed with a resin 24 (FIG. 1b) to obtain a TAB substrate 20a. Only a TAB substrate which has passed an inspection is mounted on a glass substrate 25a of a display panel 25 by heating an anisotropic adhesive agent 26 by a heating jig 27 (FIG. 1c). According to the TAB method, the TAB substrate 20a can be inspected after the IC chip 20 is connected with the film 22a and is sealed with the resin 24. Therefore, only a satisfactory TAB substrate with no defects can be mounted on the display panel 25. As a result, the completed display panel 25 has few defects attributable to the IC chip 20. Moreover, the IC chip 20 is mounted on the display panel 25 through the connecting terminal of the glass substrate 25a and the connecting terminal 28 of the tape carrier 22, which requires a small connecting area. On the other hand, the TAB method has the following disadvantages:
(1) A large number of parts are necessary including the tape carrier 22 formed of an expensive material such as polyimide and a printed circuit board.
(2) The method requires a lot of processes.
(3) Since the pattern processing of cutting the tape carrier 22 is limited and the electrode of the TAB substrate 20a and the display panel 25 are connected under the restriction of the resolution, the connection cannot be done in the case when the connecting pitch is microscopic (practically, 100 .mu.m or less).
(4) Since the IC chip 20 and the display panel 25 are connected by the connecting terminal 28, the wiring is lengthened to increase a parasitic capacitance, thereby resulting in a low operating speed.
(5) An increased number of the electrodes of the TAB substrate 20a heightens the unstableness of the connection for the inspection and undesirably increases the area of electrodes to be inspected. The impossibility of dealing with the microscopic connecting pitches and the low operating speed are especially serious. For these reasons, the TAB method cannot be applied for a high precision, high density LCD device.
According to the COG method, the connecting terminal of the glass substrate of the display panel and the connecting terminal of the IC chip are directly connected. FIG. 2a through 2d illustrate an example of the COG method. As is shown in FIG. 2a, a display panel 30 includes a pair of glass substrates 30a and 30b and a liquid crystal 30c sandwiched therebetween. An ITO (indium tin oxide) terminal 30d for connecting a driving IC chip 31 to the display panel 30 is formed in advance on a peripheral portion of the glass substrate 30a. A connecting terminal 31a of the IC chip 31 is coated with a conductive adhesive agent 32. Then, the ITO terminal 30d and the connecting terminal 31a are positionally aligned (FIG. 2b), and are connected (FIG. 2c). After that, a sealing resin 33 is filled between the IC chip 31 and the glass substrate 30a (FIG. 2d).
FIG. 3 illustrates a connecting section of the IC chip 31. The IC chip 31 has an aluminum (Al) pad 31b thereon. The Al pad 31b is covered with a passivation film 31c formed of silicon nitride (Si.sub.3 N.sub.4) except for an opening. The Al pad 31b is superposed by the connecting terminal 31a (formed of gold) through a copper layer 31d for coating. The connecting terminal 31a is adhered on the ITO film 30d on the glass substrate 30a with the conductive adhesive agent 32.
FIGS. 4a through 4e illustrate processes of producing the connecting section of the IC chip 31 by the COG method. The Al pad 31b is formed on a specified position of a substrate 31e of the IC chip 31. On the Al pad 31b, the passivation film 31c formed of Si.sub.3 N.sub.4 is formed (FIG. 4a). The Al pad 31b is etched to make the opening. The copper layer 31d for coating is formed on the Al pad 31b (FIG. 4b). The whole surface of the copper layer 31d is coated with a photo-resist film 31f, and exposed and developed to make an opening in the photo-resist film 31f above the opening of the Al pad 31b (FIG. 4c). Then, the photo-resist film 31f having the opening is coated with the connecting terminal 31a (FIG. 4d), and the photo-resist film 31f is removed (FIG. 4e). In the connecting section obtained in this way, an opening made by removing the photo-resist film 31f has a thickness of approximately 80 .mu.m, and the connecting terminal 31a has a thickness of approximately 50 .mu.m. The connecting section has a diameter of approximately 170 .mu.m. Although a smaller diameter of the connecting section can be realized by increasing the thickness of the photo-resist film 31f, it is usually difficult to increase the thickness of the photo-resist film 31f to 5 .mu.m or thicker. As a result, the connecting section cannot be very small.
The COG method has the following advantages:
(1) A small number of necessary parts and a simple procedure lowers the production cost, which is especially effective since the number of the driving IC chips increases in accordance with an increase of the number of the pixels in the LCD device.
(2) The direct connection of the IC chip on the glass substrates serves to produce a thin LCD device.
(3) The yielding ratio and the quality of the IC chip is enhanced due to a small number of connecting positions.
On the other hand, the COG method has the following disadvantages:
(1) The connecting terminals on the IC chip and the glass substrate are damaged due to a temperature change since silicon forming the IC chip and the glass substrate have different coefficients of thermal expansion. While the coefficient of thermal expansion of silicon is 3.5.times.10.sup.-6 /.degree.C., that of glass is approximately 5.0 to 7.0.times.10.sup.-6 /.degree.C. The connecting section is possibly cracked by heightening and lowering the temperature in repetition, thus resulting in a low reliability.
(2) The sealing resin is filled between the IC chip and the glass substrate in order to minimize the affects of the thermal stress caused by the above difference of the coefficient of thermal expansion. This requires a space between the IC chip and the glass substrate, which requires the connecting section to be thicker. Practically, in the case when the thickness of the connecting section is 50 .mu.m, the diameter thereof becomes 170 .mu.m. Accordingly, a large area is necessary for connection.
(3) Since the ITO terminal is connected to the glass substrate, the IC chip cannot be processed using the microscopic processing technology. Due to a low density of the connecting terminals caused by the above reason, the method cannot be used for a high density LCD device having a pitch between pixels of several tens of micrometers. The low density of the connecting terminal also enlarges the area for connecting the IC chip and the glass substrate.
The above-mentioned three disadvantages are all serious. The excessive thermal stress lowers the reliability of the display device and deteriorates the characteristics of the IC chip. Due to the impossibility of microscopic processing and the necessity of a large connection area, the COG method cannot be used for an LCD device which is microscopic and has a high density. Moreover, the glass substrate has an inferior heat radiation. While silicon has a thermal conductivity of 123 W/m.multidot.K, and aluminum has that of 238 W/m.multidot.K, glass has that of approximately 1.2 W/m.multidot.K. In other words, the thermal conductivity of glass is smaller than those of silicon and aluminum by two digits. Accordingly, in the case when the temperature of the parts on the glass substrate and the liquid crystal is increased, the heat is hardly radiated through the glass substrate. As a result, in the case when temperature is a problem, such as when heat is generated in the silicon IC or a high intensity light is radiated to the LCD device, a special cooling device is necessary.